1. Field of the Invention
The present invention relates to a high-toughness silicon carbide composite ceramic which is capable of being machined by electrical discharge, and which is applicable to products such as engine parts of a gas turbine and rotor parts of a turbo-charger that are required .+-.o have a resistance to high temperature at a temperature of not lower than 1,400.degree. C.
2. Description of the Related Art
A silicon carbide (hereinunder referred to as "SiC") material has a higher resistance to high temperature than a silicon nitride material and, in particular, in an SiC material with a sintering aid except oxides, such as boron carbide and aluminum nitride, added thereto, the strength is not lowered even at a temperature not lower than 1,500.degree. C. On the other hand, the most serious weak point of an SiC material is that the fracture toughness value and the resistance to mechanical shock are low. An SiC material ordinarily has a K.sub.IC value, which represents a fracture toughness, of 2 to 4 MPa.multidot.m.sup.1/2, which is about half that of a silicon nitride material, namely, 5 to 7 MPa.multidot.m.sup.1/2. Therefore, a turbine rotor manufactured from an SiC material involves the fatal defect that a blade is apt to be broken by foreign particles which are flown thereto.
On the other hand, in order to enhance the toughness of a ceramic material, an attempt has been made to add SiC whiskers thereto as a reinforcing material. For example, an SiC whisker reinforced alumina composite ceramic was manufactured by Wei et al [G. C. Wei and P. F. Becher, Am. Ceram. Soc. Bull., 64, No. 2, pp 298 to 304 (1985)], and an SiC whisker reinforced silicon nitride composite ceramic was manufactured by Ueno et al [Kazuo Ueno and Yasuo Toibata, J. Ceram. Soc. Japan, 91, No. 11, pp 491 to 497 (1983)]. These ceramic materials obtained higher fracture toughness values than those without whiskers.
However, although these composite ceramic materials have improved fracture toughness values, they have restrictions in the high-temperature resistance as a matrix material, and insufficient in the high-temperature strength at a temperature of not lower than 1,400.degree. C, which is aimed at by the present invention.